An interrupt is generated when the UART has finished transmitting or
receiving a byte. The interrupt handling routines use circular buffers
for buffering received and transmitted data.

Setting up

The UART_RXn_BUFFER_SIZE and UART_TXn_BUFFER_SIZE symbols define
the size of the circular buffers in bytes. These values must be a power of 2.
You may need to adapt this symbols to your target and your application by adding into your compiler options:

-DUART_RXn_BUFFER_SIZE=nn -DUART_TXn_BUFFER_SIZE=nn

RXn and TXn refer to the UART number, for UART3 with 128 byte buffers, add:

-DUART_RX3_BUFFER_SIZE=128 -DUART_TX3_BUFFER_SIZE=128

UART0 is always enabled by default, to enable the other available UARTs, add the following to your compiler’s symbol options for the relevant UART (also known as USART) number.

-DUSART1_ENABLED -DUSART2_ENABLED -DUSART3_ENABLED

To enable large buffer support (over 256 bytes, up to 2^15 bytes) use:

Notes

Buffer overflow behaviour

When the RX circular buffer is full, and it receives further data from the UART, a buffer overflow condition occurs. Any new data is dropped. The RX buffer must be read before any more incoming data from the UART is placed into the RX buffer.

If the TX buffer is full, and new data is sent to it using one of the uartN_put*() functions, this function will loop and wait until the buffer is not full any more. It is important to make sure you have not disabled your UART transmit interrupts (TXEN*) elsewhere in your application (e.g with cli()) before calling the uartN_put*() functions, as the application will lock up. The UART interrupts are automatically enabled when you use the uartN_init() functions. This is probably not the idea behaviour, I’ll probably fix this some time.

For now, make sure TXEN* interrupts are enabled when calling uartN_put*() functions. This should not be an issue unless you have code elsewhere purposely turning it off.